The disclosures of the following priority applications are incorporated herein by reference:
Japanese Patent Application No. 2000-261362 filed Aug. 30, 2000;
Japanese Patent Application No. 2000-395274 filed Dec. 26, 2000.
Japanese Patent Application No. 2001-251972 filed Aug. 22, 2001;
Japanese Patent Application No. 2001-251980 filed Aug. 22, 2001;
1. Field of the Invention
The present invention relates to an imaging optical system capable of forming an image of an object arranged at a finite distance, as in a scanner optical system, and more particularly relates to an imaging optical system having superior optical performance with the improved correction of chromatic aberration over a wide wavelength range extending from the visible wavelength range to the infrared wavelength range.
2. Description of Related Art
An optical system for a scanner requires the ability to faithfully read the information of the original picture or object being scanned. Accordingly, it is necessary to correct various aberrations like spherical aberration for a single color, as well as to satisfactorily correct longitudinal and lateral chromatic aberrations. Longitudinal chromatic aberration increases proportional to the square of the imaging magnification in an optical system that forms the image of an object arranged at a finite distance, such as in an optical system for a scanner. Thus, the correction of chromatic aberration is even more critical.
Generally, it is necessary with an optical system for a scanner to faithfully reproduce the original picture or object in the visible wavelength range. However, it has also become necessary in recent years for such optical system having superior optical performance with the improved correction of chromatic aberration over a wide wavelength range extending from the visible wavelength range to the infrared wavelength range in the vicinity of 850 nm.
With increasing the magnification, longitudinal chromatic aberration increases proportional to the square of the imaging magnification. It becomes difficult to obtain such optical system having superior optical performance, and, in particular, difficult to correct chromatic aberration.
When longitudinal chromatic aberration is not sufficiently corrected, the best focus positions (where the best optical performance on the axis can be obtained) for three-wavelength ranges (red, green, and blue) shift along the optical axis with each other. Accordingly, when a CCD is arranged at the best focus position for a blue wavelength range, sufficient optical performance may not be obtained for a green wavelength range.
On the other hand, when lateral chromatic aberration is not sufficiently corrected, the height from the optical axis where the image of an original is formed varies in accordance with the wavelength. Accordingly, the image of the original is recorded with different dimension in each wavelength.
Image information recorded on a film is observed in a state magnified and projected on a printing paper or the like. Therefore, when image information recorded on a film is input to a computer by means of a photoelectric converter or the like, a scanner optical system is required to have a high resolving power. Since a scanner optical system actually has a high resolving power, the high resolving power permits recording even a minute dust on a film as image information.
In this case, the minute dust on the film can be detected by using, for example, near infrared wavelength range. With removing the detected image of the dust from the image information by means of electrical processing, the image information recorded on the film can be input excellently even a dust is stuck on the film.
However, in a scanner optical system up to now, chromatic aberration for near infrared wavelength has not been sufficiently corrected, so that when a dust stuck on a film is detected with near infrared wavelength, the image of the dust being recorded by a photoelectric converter becomes blurred. As a result, with removing the dust image from image information obtained with red, green and blue wavelength regions, the effect of removing the dust image is not sufficient, so that it is inconvenient that image information cannot be input excellently.
The present invention is made in view of the aforementioned problem and has an object to provide an imaging optical system capable of forming an image of an object arranged at a finite distance having superior optical performance with the improved correction of chromatic aberration over a wide wavelength range extending from the visible wavelength range to the infrared wavelength range.
According to one aspect of the present invention, an imaging optical system includes, in order from an object, a front lens group with a positive refractive power having at least one lens element, an aperture stop, and a rear lens group with a positive refractive power having at least one lens element. The front lens group includes a double convex positive lens arranged to the most object side. At least one of the front lens group and the rear lens group includes a triple cemented lens being adjacent to the aperture stop having a negative refractive power as a whole composed of a first positive lens, a negative lens, and a second positive lens. The following conditional expression is satisfied;
0.3 less than |fs|/f less than 5.0xe2x80x83xe2x80x83(1)
where fs denotes the focal length of the cemented lens at e-line (xcex=546.07 nm) and f denotes the focal length of the imaging optical system at e-line (xcex=546.07 nm).
In one preferred embodiment of the present invention, the cemented lens has a positive lens made of a glass material having Abbe number xcexdd of 65 or more and also the partial dispersion ratio P of 0.8 or more. The cemented lens has a meniscus lens arranged closest to the aperture stop having a strong powered surface facing to the opposite side of the space where the aperture stop exists. The following conditional expression is satisfied;
0.1 less than LD/(|xcex2|xc2x7f) less than 4xe2x80x83xe2x80x83(2)
where LD denotes the total sum of the thickness along the optical axis of the positive lenses made of the glass material having Abbe number xcexdd of 65 or more and also the partial dispersion ratio P of 0.8 or more, P denotes the partial dispersion ratio (ngxe2x88x92ne)/(nFxe2x88x92nC), ng denotes refractive index at g-line (xcex=435.84 nm), ne denotes refractive index at e-line (xcex=546.07 nm), nF denotes refractive index at F-line (xcex=486.13 nm), nC denotes refractive index at C-line (xcex=656.27 nm), and xcex2 denotes the imaging magnification of the imaging optical system at e-line (xcex=546.07 nm).
In one preferred embodiment of the present invention, at least two positive lenses, whose glass material has Abbe number xcexdd of 65 or more and also the partial dispersion ratio P of 0.8 or more, are arranged to both object side and image side of the aperture stop. The following conditional expressions are satisfied;
0.03 less than "PHgr"R/(|xcex2|xc2x7|fs|) less than 3xe2x80x83xe2x80x83(3)
xe2x88x920.01 less than LA/f less than 0.01xe2x80x83xe2x80x83(4)
where "PHgr"R denotes the effective diameter of the most-image-side lens, and LA denotes an amount of longitudinal chromatic aberration of the imaging optical system at s-line (xcex=852.11 nm) as measured with respect to an e-line reference wavelength.
In one preferred embodiment of the present invention, a first and a second cemented lenses having a negative refractive power as a whole are arranged adjacent to both sides of the aperture stop. Each of the first cemented lens and the second cemented lens is a triple cemented lens composed of a first positive lens, a negative lens, and a second positive lens. The most aperture stop side lens of each cemented lens is a positive meniscus lens. The following conditional expression is satisfied;
0.5 less than fs1/fs2 less than 3.0xe2x80x83xe2x80x83(5)
where fs1 denotes the focal length at e-line (xcex=546.07 nm) of the first cemented lens arranged to the object side of the aperture stop and fs2 denotes the focal length at e-line (xcex=546.07 nm) of the second cemented lens arranged to the image side of the aperture stop.
According to another aspect of the present invention, an imaging optical system includes, in order from an object, a front lens group having a positive refractive power, an aperture stop, and a rear lens group having a positive refractive power. The front lens group includes, in order from the object, a first lens element having a positive refractive power, and a second lens element having a negative refractive power cemented with at least two lens elements composed of positive and negative lenses wherein the second lens element has a meniscus shape having a concave surface facing to the image side. The rear lens group includes, in order from the object, a third lens element having a negative refractive power cemented with at least two lens elements composed of positive and negative lenses wherein the third lens element has a meniscus shape having a concave surface facing to the object side, and a fourth lens element having a positive refractive power with a meniscus shape having a concave surface facing to the object side. The following conditional expressions are satisfied;
xe2x88x920.01 less than RSA/f less than 0.01xe2x80x83xe2x80x83(9)
xe2x88x920.0085 less than LAM/f less than 0.0085xe2x80x83xe2x80x83(10)
where RSA denotes the maximum value of spherical aberration at e-line, LAM denotes the maximum value of longitudinal chromatic aberration of the imaging optical system between 435.8 nm and 1014 nm as measured with respect to an e-line reference wavelength, and f denotes the focal length of the imaging optical system at e-line.
In one preferred embodiment of the present invention, a positive lens made of a glass material satisfying the following conditional expressions is used at least at one position among the two lens groups between which the aperture stop of the imaging optical system is located;
65 less than xcexdxe2x80x83xe2x80x83(11)
1.40 less than nd less than 1.65xe2x80x83xe2x80x83(12).
where xcexdd denotes Abbe number and nd denotes refractive index at d-line (xcex=587.6 nm).
In one preferred embodiment of the present invention, a negative lens made of a glass material satisfying the following conditional expressions is used at least at one position among the two lens groups between which the aperture stop of the imaging optical system is located;
35 less than xcexdd less than 45xe2x80x83xe2x80x83(13)
1.60 less than nd less than 1.70xe2x80x83xe2x80x83(14)
where xcexdd denotes Abbe number and nd denotes refractive index.
In one preferred embodiment of the present invention, when the second lens element is composed of three lens elements, the following conditional expression is satisfied;
0 less than xe2x88x92f2b/f less than f2a/f less than f2c/fxe2x80x83xe2x80x83(15)
where f2a denotes the focal length of a lens element at e-line located to the most object side of the second lens element, f2b denotes the focal length of a lens element at e-line located at the middle of the second lens element, f2c denotes the focal length of a lens element at e-line located to the most image side of the second lens element, and f denotes the focal length of the imaging optical system at e-line.